The present invention relates to a gallium nitride-based compound semiconductor device, particularly, to a gallium nitride-based compound semiconductor laser.
In recent years, gallium nitride-based compound semiconductor materials such as GaN, InGaN, GaAlN, and InGaAlN attract attentions as materials of a blue semiconductor laser. The semiconductor lasers formed of these compound semiconductor materials emit light having a short wavelength, making it possible to draw the emitted laser beam to have a small diameter. Therefore, these lasers are expected to provide an excellent light source for a high density information processing apparatus such as an optical disk.
In general, magnesium is used as a p-type impurity, i.e., dopant, in a gallium nitride-based compound semiconductor. This impurity has a deep acceptor level and, thus, is low in activation rate, with the result that only one above several tens to one above several hundreds of the doped impurity acts as an effective p-type carrier. If the dose of the impurity is excessively increased in an attempt to increase the carrier density, the quality of the p-type semiconductor crystal is impaired. It is also reported that an excessively large dose amount inversely leads to reduction in the carrier density.
Under the circumstances, it is essentially impossible to obtain a p-type layer of a low resistivity in a gallium nitride-based semiconductor device using magnesium as a p-type impurity. It should also be noted that the number of p-type carriers, i.e., holes, in the p-type layer is markedly smaller than that of n-type carriers, i.e., electrons, in the n-type layer, giving rise to a carrier overflow phenomenon that the n-type carriers overflow the active layer to reach the p-type layer. Such being the situation, it is difficult to use a gallium nitride-based compound semiconductor material for manufacturing a semiconductor device having a low threshold value, operable at a low voltage, exhibiting a high reliability and, thus, adapted for use in, for example, an optical disk. Also, a semiconductor device adapted for an optical arithmetic calculation such as a short wave bistable type or self-excitation type semiconductor device has not yet been developed.